The following account of the prior art relates to one of the areas of application of the present application, hearing aids. A hearing aid comprises an input transducer (microphone) and an output transducer (speaker) and a forward path there between providing a frequency dependent gain of a signal originating from the input transducer and presenting such resulting signal to the output transducer.
Frequency dependent acoustic, electrical and mechanical feedback identification and compensation methods are commonly used in listening devices, in particular hearing instruments (HI), to ensure their stability. Unstable systems due to acoustic feedback tend to significantly contaminate the desired audio input signal with narrow-band frequency components, which are often perceived by a user as howl or whistle. Acoustic feedback occurs because the output loudspeaker signal from an audio system providing amplification of a signal picked up by a microphone is partly returned to the microphone via an acoustic coupling through the air or other media. The part of the loudspeaker signal returned to the microphone is then re-amplified by the system before it is re-presented at the loudspeaker, and again returned to the microphone. As this cycle continues, the effect of acoustic feedback becomes audible as artifacts or even worse, howling, when the system becomes unstable. The problem appears typically when the microphone and the loudspeaker are placed closely together, as e.g. in hearing aids. Some other classic situations with feedback problem are telephony, public address systems, headsets, audio conference systems, etc.
Feedback performance of hearing instruments becomes more and more important, since fittings get more and more open in that an ear canal part of the instrument does not fully close the ear canal so that sound intended for the ear drum more easily escapes, resulting in higher feedback likelihood. This is why hearing instrument manufacturers have been developing technology to fight feedback, e.g. mechanical designs with optimized vibration performance, but also electronic systems that can actively fight feedback—so called “Feedback Cancellers”.
The verification of feedback performance for a given hearing instrument system is difficult, because realistic feedback only occurs if the system is integrated as a whole and used in realistic wearing conditions. Especially, the so-called (acoustic) feedback path (the acoustic path from the hearing instrument's sound outlet to its microphone inlet) has to be estimated with realistic parameters:                The throughput delay of the path has to be in a realistic parameter range, because usually Feedback Cancellers in hearing aids are based on certain assumptions on this parameter range that should preferably not be violated during the majority of test cases. Thus the test system preferably comprises an appropriate variable delay unit.        Ideally, the test system is adapted to be able to vary the transfer function of the feedback path within parameter ranges given by the realistic use of the hearing instrument when being worn at the ear of a user. To cover enough realistic situations with the test, means to enforce a set of different transfer functions of the feedback path should preferably be included in the test system. To cover all elements of this set, feedback-related testing requires means (e.g. appropriate filter elements) of enforcing a desired transfer function within a certain tolerance range.        
A realistic condition could be emulated by putting the HI on an artificial ear at an artificial head; however, the systematic variation of the feedback path cannot be achieved in this setup with prior art components.